Question

A simple electric motor has an armature resistance of $$1\,\Omega $$  and runs from a dc source of 12 volt. When running unloaded it draws a current of $$2\,amp.$$   When a certain load is connected, its speed. becomes one-half of its unloaded value. What is the new value of current drawn?

A. $$7\,A$$  
B. $$3\,A$$
C. $$5\,A$$
D. $$4\,A$$
Answer :   $$7\,A$$
Solution :
Let initial e.m.f. induced $$= e.$$
$$\therefore $$ Initial current $$i = \frac{{E - e}}{R}$$
$${\text{i}}{\text{.e}}{\text{.,}}\,2 = \frac{{12 - e}}{1}$$
This gives $$e = 12 - 2 = 10\,{\text{volt}}{\text{.}}$$     As $$e \propto \omega .$$  when speed is halved, the value of induced e.m.f. becomes
$$\frac{e}{2} = \frac{{10}}{2} = 5\,{\text{volt}}$$
$$\therefore $$ New value of current
$$i' = \frac{{E - e}}{R} = \frac{{12 - 5}}{1} = 7\,A$$

Releted MCQ Question on
Electrostatics and Magnetism >> Electromagnetic Induction

Releted Question 1

A thin circular ring of area $$A$$ is held perpendicular to a uniform magnetic field of induction $$B.$$ $$A$$ small cut is made in the ring and a galvanometer is connected across the ends such that the total resistance of the circuit is $$R.$$ When the ring is suddenly squeezed to zero area, the charge flowing through the galvanometer is

A. $$\frac{{BR}}{A}$$
B. $$\frac{{AB}}{R}$$
C. $$ABR$$
D. $$\frac{{{B^2}A}}{{{R^2}}}$$
Releted Question 2

A thin semi-circular conducting ring of radius $$R$$ is falling with its plane vertical in horizontal magnetic induction $$\overrightarrow B .$$  At the position $$MNQ$$  the speed of the ring is $$v,$$ and the potential difference developed across the ring is
Electromagnetic Induction mcq question image

A. zero
B. $$\frac{{Bv\pi {R^2}}}{2}$$  and $$M$$ is at higher potential
C. $$\pi RBv$$  and $$Q$$ is at higher potential
D. $$2RBv$$  and $$Q$$ is at higher potential
Releted Question 3

Two identical circular loops of metal wire are lying on a table without touching each other. Loop-$$A$$ carries a current which increases with time. In response, the loop-$$B$$

A. remains stationary
B. is attracted by the loop-$$A$$
C. is repelled by the loop-$$A$$
D. rotates about its $$CM,$$  with $$CM$$  fixed
Releted Question 4

A coil of inductance $$8.4 mH$$  and resistance $$6\,\Omega $$  is connected to a $$12 V$$  battery. The current in the coil is $$1.0 A$$  at approximately the time

A. $$500 s$$
B. $$25 s$$
C. $$35 ms$$
D. $$1 ms$$

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Electromagnetic Induction


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